This invention relates to protective time-delay circuits, and is more especially directed to circuits which delay the energization of an electrical device until a predetermined delay time has passed after a previous deenergization. These circuits are known as anti short-cycle timers.
It is common in air conditioning circuits to protect the ac motor of the air conditioner compressor with a delay-on-make time delay device. When the compressor has been running, a pressure head builds up. This head bleeds off slowly after the compressor stops. The presence of the head produces a reverse torque on the compressor until the head has decayed. Attempting to restart the compressor motor in the presence of the pressure head can result in stall and overheating, or can result in the motor running in reverse. The danger of reverse running is especially problematic for scroll compressor units. The delay circuit holds up energization of the compressor for a time long enough for the pressure head to drop, and then closing to energize the motor. This delay time is typically one to several minutes.
Unfortunately, a delay-on-make protective circuit will delay power-up of the motor not only after it has been shut off recently, but also when the motor has been off for an extended time and no pressure head exists. This produces unnecessary delays when there is a call for cooling. For example, an individual entering an uncomfortably warm or stuffy room, and turning the thermostat to a cooling position will find that the air conditioner does not respond because of this built-in delay. Then, the individual as often as not will attempt to reset the thermostat. However, each time he or she does so, the delay time is reinitiated, further delaying the onset of cooling.
Other attempts at protective circuits for this purpose involve a delay timer that commences when a motor or load device is switched off, so that a delay in operation will be experienced only if the motor or load device is turned off and then turned back on immediately. However, these devices are invariably quite complex, involving numerous discrete elements, and typically including four or more transistors, numerous capacitors, zeners, and resistors, which make the devices difficult to design and expensive to build.
Another problem unsolved is that of bounce. That is, if a mercury thermostat is used, each time the thermostat turns on, the ball of mercury in it initially makes intermittent contact with the thermostat contacts. This makes and breaks contact a number of times, which initiates and breaks the timing of the protective delay circuit. Consequently, there is invariably a delay in cooling after a call for cooling, even when there has not been recent compressor operation.